1. Field of the Invention
This invention relates to a process for producing a polymerization toner used in, e.g., electrophotography, electrostatic recording, magnetic recording and toner jet recording.
2. Related Background Art
A number of methods are conventionally known as electrophotography. In general, copied images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat and/or pressure. As methods for developing electrostatic latent images by the use of toners or methods for fixing toner images, a variety of methods have been proposed.
Toners used for such purpose have commonly been produced by melt-kneading colorants such as dyes and/or pigments into thermoplastic resins to effect dispersion uniformly, followed by pulverization by means of a fine grinding mill and then classification of the pulverized product to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a production method, but there is a certain limit, i.e., a limit to the range in which toner materials are selected. For example, resin-colorant dispersions must be brittle enough to be pulverizable by means of economically available production apparatus. However, resin-colorant dispersions made brittle in order to meet these requirement tend to result in a broad particle size range of the particles formed when actually pulverized at a high speed, especially causing the problem that fine particles tend to be included in the particles in a relatively large proportion. Moreover, such highly brittle materials tend to be further pulverized or powdered when used in development in, e.g., copying machines.
In this method, it is also difficult to perfectly uniformly disperse solid fine particles of colorants or the like in the resin, and, depending on the degree of their dispersion, toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency when images are formed. Accordingly, care must well be taken when colorants are dispersed. Also, colorants may come bare to rupture sections of toner particles, and may cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, various polymerization toners and methods of producing such toners are proposed, including toners produced by suspension polymerization as disclosed in Japanese Patent Publications No. 36-10231, No. 43-10799 and No. 51-14895. For example, in the suspension polymerization, a polymerizable monomer, a colorant and a polymerization initiator, and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a monomer composition. Thereafter, this monomer composition is dispersed in a continuous phase, e.g., an aqueous phase, containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization at all, the toner particles are not required to be brittle, and hence soft materials can be used. Also, colorants by no means come bare to the surfaces of toner particles, and hence the toner can have a uniform triboelectric charging performance. This method has such advantages. Also, since the toner obtained has a relatively sharp particle size distribution, the step of classification can be omitted, or even when classification is carried out, the toner can be obtained in a high yield.
In order to cause no toners to adhere to the surface of the fixing roller, a measure has also been hitherto taken such that the roller surface is formed of a material such as silicon rubber or fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a high releasability as exemplified by silicone oil or fluorine oil. However, this method, though very effective in view of the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence as a matter of course has the problem such that the fixing assembly must be complicated complicated. Also, this application of oil is involved in the difficulty that it causes separation of layers constituting the fixing roller to consequently acceleratedly shorten the lifetime of the fixing roller.
Accordingly, based on the idea that the fluid for preventing offset should be fed from the interiors of toner particles at the time of heat fixing without use of, e.g., any device for feeding silicone oil, a method has been proposed in which a release agent such as a low-molecular weight polyethylene or a low-molecular weight polypropylene is incorporated into toner particles.
It is known that a wax is incorporated as a release agent into toner particles. For example, this is disclosed in Japanese Patent Publications No. 52-3304 and No. 52-3305 and Japanese Patent Application Laid-open No. 57-52574.
Japanese Patent Applications Laid-open No. 3-50559, No. 2-79860, No. 1-109359, No. 62-14166, No. 61-273554, No. 61-94062, No. 61-138259, No. 60-252361, No. 60-252360 and No. 60-217366 disclose incorporation of waxes in toners.
Waxes are used for the purpose of improving anti-offset properties at the time of low-temperature fixing or high-temperature fixing of toners or improving fixing performance at the time of low-temperature fixing, but on the other hand tend to cause a lowering of anti-blocking properties of toners, a lowering of developing performance because of temperature rise in copying machines, or a lowering of developing performance because of migration of wax toward toner particle surfaces when toners are left for a long term.
As a countermeasure for the above problems, toners produced by suspension polymerization are proposed. For example, according to the disclosure in Japanese Patent Application Laid-open No. 5-341573, a polar component is added to a monomer composition, where components having polar groups, contained in the monomer composition, tend to become present at surface layer portions which are interfaces with the aqueous phase and non-polar components do not tend to become present at the surface layer portions, and hence toner particles can have core/shell structure.
In the toner produced by suspension polymerization, the wax is encapsulated in toner particles. This enables achievement of both the anti-blocking properties and the high-temperature anti-offset properties that conflict with each other, and also enables prevention of high-temperature offset without applying any release agent such as oil to fixing rollers.
As also disclosed in Japanese Patent Publications No. 7-82248 and No. 7-120072, as a production process intended to improve fixing performance of polymerization toners, it is proposed to effect granulation of a monomer composition in an aqueous medium and thereafter add a polymerization initiator to the aqueous medium to carry out suspension polymerization. This method makes it possible to make toner particles spherical, make them have a sharp particle size distribution and also incorporate therein the wax in a large quantity.
As still also disclosed in Japanese Patent Application Laid-open No. 10-239900, it is proposed to disperse a monomer composition in an aqueous medium and thereafter add a polymerization initiator to the aqueous medium, followed by further dispersion to prepare droplets to carry out suspension polymerization.
In addition, it has become popular to use copying machines or printers for forming full-color images.
In the case of fixing assemblies in full-color image-forming apparatus, a plurality of toner layers corresponding to magenta toner, cyan toner, yellow toner and black toner are formed on a transfer medium, and hence the offset tends to occur because of toner layers formed in large thickness.
As transfer mediums on which toner images are fixed, paper of various types, coated paper, plastic films and so forth are commonly used. In particular, a need for transparency films (OHP films) has increased, which make use of an overhead projector for its presentation. Especially in OHP films, as different from paper, a large quantity of oil is present on the OHP film surface after fixing, because of their low oil absorption capacity. Silicone oil may evaporate by heat to contaminate the interior of image forming apparatus, and also has the problem of disposal of recovered oil.
However, taking account of the recent demand for small size, light weight and high reliability, it is preferable also in the full-color image-forming apparatus to omit even such a supplementary device.
In order to improve color-mixing performance of toners and also provide toners having superior low-temperature fixing performance, it is preferable for binder resins to melt instantaneously at the time of fixing. However, binder resins having such properties on the one hand may on the other hand necessarily have poor high-temperature anti-offset properties, anti-blocking properties and running performance.
An object of the present invention is to provide a process for producing a polymerization toner which can meet the requirements stated above.
Another object of the present invention is to provide a process for producing a polymerization toner having a superior fixing performance.
Still another object of the present invention is to provide a process for producing a polymerization toner having a superior continuous productivity.
A further object of the present invention is to provide a process for producing a polymerization toner promising a good charge quantity and having superior developing performance (toner charge quantity, image density) and transfer performance even in many-sheet running.
The present invention provides a process for producing a polymerization toner, which comprises preparing a polymerizable-monomer composition which contains at least a polymerizable monomer and a colorant and does not contain any polymerization initiator, introducing the polymerizable-monomer composition into an aqueous medium to start granulation, adding a polymerization initiator to the aqueous medium in the course of the granulation, and then polymerizing the polymerizable-monomer composition having been granulated, to produce toner particles;
the polymerization initiator being added to the aqueous medium over a period of from 5 seconds to 300 seconds, and the time T (second) taken to add the polymerization initiator and the number N of pass times per unit time (times/second) of a stirring blade used in the granulation satisfying the relationship of:
3xe2x89xa6Txc3x97Nxe2x89xa6500. 
In another embodiment, the present invention provides a process for producing a polymerization toner, which comprises preparing a polymerizable-monomer composition which contains at least a polymerizable monomer and a colorant and does not contain any polymerization initiator, introducing the polymerizable-monomer composition into an aqueous medium to effect granulation, adding a polymerization initiator to the aqueous medium after the granulation has been completed, and then polymerizing the polymerizable-monomer composition having been granulated, to produce toner particles;
the polymerization initiator being added to the aqueous medium over a period of from 5 seconds to 300 seconds, and the time T (second) taken to add the polymerization initiator and the number N of pass times per unit time (times/second) of a stirring blade used in the reaction satisfying the relationship of:
5xe2x89xa6Txc3x97Nxe2x89xa62,500. 
In still another embodiment, the present invention provides a process for producing a polymerization toner, which comprises preparing a polymerizable-monomer composition which contains at least a polymerizable monomer and a colorant and does not contain any polymerization initiator, adding a polymerization initiator to an aqueous medium, introducing the polymerizable-monomer composition into the aqueous medium to effect granulation, and then polymerizing the polymerizable-monomer composition having been granulated, to produce toner particles;
the polymerizable-monomer composition being introduced into the aqueous medium within 10 minutes after the polymerization initiator has been added, and the time T (second) taken to add the polymerization initiator being:
5.0xc3x9710xe2x88x925xe2x89xa6T/txc2xdxe2x89xa61.0xc3x9710xe2x88x922 
where txc2xd is the half-life period of the polymerization initiator at granulation temperature.